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Dynamic Extraction of Spearmint Oil Components byUsing Supercritical CO2

KYOUNG HEON KIM* and JUAN HONGDEPARTMENT OF CHEMICAL AND BIOCHEMICAL ENGINEERING AND MATERIALS SCIENCE

UNIVERSITY OF CALIFORNIA, IRVINE

IRVINE, CALIFORNIA 92697, USA

ABSTRACT

The effects of various extraction conditions on the dynamic extraction of the es-

sential oil components carvone and limonene from spearmint leaves using SC-CO2were investigated. The extraction rate increased with increasing pressure or decreas-

ing temperature. An increase of the CO2 flow rate increased the extraction rate but de-

creased the solvent efficiency of CO2. Ground leaf samples with a smaller particle size

showed an enhanced initial extraction rate for carvone as compared to larger particle

size leaf samples. The use of an ethanol modifier did not enhance the extraction rate

but did cause the coextraction of pigment and waxy substances.

Key Words. Extraction; Supercritical carbon dioxide; Spearmint oil; Car-

vone; Limonene

INTRODUCTION

Solvent extraction and steam distillation have been conventionally used to

isolate essential oils from plant materials for industry. Supercritical CO2 (SC-

CO2) has been considered as an alternative solvent due to its nontoxicity, non-flammability, relatively moderate critical point, and low cost even at high pu-

SEPARATION SCIENCE AND TECHNOLOGY, 35(2), pp. 315322, 2000

Copyright 2000 by Marcel Dekker, Inc. www.dekker.com

315

* Current address: National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO

80401, USA.

To whom correspondence should be addressed. Telephone: (949) 824-8278. FAX: (949) 824-

2541. E-mail: [email protected]

TECHNICAL NOTE

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rity. Compared to phase equilibrium studies using SC-CO2, relatively fewer

studies are available on the extraction of natural products using SC-CO2 (1).

Since extraction from plant matrices is much different from extraction for

equilibrium solubility measurements of pure substances, knowledge of the ex-traction behavior in actual SC-CO2 extraction from plant matrices would beuseful for industrial applications.

Spearmint oil is one of the most popular flavoring materials used in foods,

pharmaceuticals, and perfumes. The major components of spearmint oil are l-

carvone and l-limonene, and the unique odor of spearmint is characterized by

l-carvone (25). Spearmint oil extraction using SC-CO2 was carried out by

Barton et al. (6) and Ozer et al. (7). However, no information exists on the

time-dependent extraction behavior and mass transfer mechanism for

spearmint oil extraction. For this paper the effects of temperature, pressure,

exiting CO2 flow rate, leaf particle size, and ethanol modifier on the SC-CO2extraction of essential oil components from spearmint leaves were studied.

EXPERIMENTAL SECTION

Materials

Spearmint leaves obtained from southern California were washed with

deionized water and air-dried at room temperature for 1 week. They were then

cut into pieces of 0.15 mm average radius. When fine particle-sized leaves

were needed, the cut leaves were ground using a mortar and pestle to reduce

their average radius to 0.03 mm. CO2 of 99.6 or 99.9% purity was used.Methanol (99.9% purity) was used as a collection solvent, and ethanol (99.9%

purity) was used as a modifier.

Apparatus and Procedure

Extraction experiments were performed with the apparatus schematically

shown in Fig. 1. The heart of the supercritical delivery system (CCS,

Unionville, NJ) is an ISCO LC-50 syringe pump with a capacity of 50 mL.

This pump was designed to compress CO2 and to deliver fluids up to 6000 psi.

A spearmint leaf sample (2 g) was placed with about 40 mL of glass beads

(2 mm diameter) in the extraction vessel to disperse the leaf particles in theSC-CO2 phase. The extractor was pressurized and heated, and when the pres-

sure and temperature of the extractor reached the desired points, the heated

flow restriction valve was opened. This marked the start of extraction.

The supercritical CO2 stream laden with essential oil left the extractor and

expanded at atmospheric pressure after the restriction valve, passing through

the flowmeter before being vented. The flow rate of the exiting CO2 was

recorded to determine the cumulative amount of CO2 passed during extrac-

316 KIM AND HONG

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tion. The essential oil extract dissolved in methanol was taken at specific time

intervals to obtain extraction curves with extraction time.

The analysis of the extract dissolved in methanol was performed by using a

Varian 3350 gas chromatograph (3% OV101 on Chromosorb packed column,

a thermal conductivity detector, and N2 as the carrier gas).

RESULTS AND DISCUSSION

Effect of Pressure

Figure 2 shows the effect of pressure on the cumulative amount of carvone

and limonene with extraction time at 39C. It is likely that an increase of the

dissolving power of CO2 which resulted from a density increase of CO2 at

higher pressure increased the extraction at higher pressure.

The extraction rate in the initial period was probably higher in comparison

with that in the latter period due to the relatively easy extraction of accessiblesolute from the leaf matrices in the initial period. In other words, the extrac-

tion rate decreased with time since relatively inaccessible solute inside the

particles needed to be extracted as extraction time passed.

The extraction at 1500 psi resulted in the coextraction of waxy materials

and green pigments along with the essential oil components. Due to the higher

dissolving power of SC-CO2 at a higher extraction pressure, such less soluble

substances could be coextracted. Similar results were reported by others

(810).

DYNAMIC EXTRACTION OF SPEARMINT OIL COMPONENTS 317

FIG. 1 Schematic diagram of the experimental apparatus: (1) CO2 cylinder; (2) fluid delivery

system; (3) valve oven; (4) extraction vessel; (5) heating tape; (6) temperature controller; (7)

thermocouple; (8) pressure gauge; (9) flow restriction valve; (10) collection solvent; (11)

collection vessel; (12) ice bath; (13) flowmeter.

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Effect of Temperature

To find the effect of extraction temperature, extraction was carried out at 39

and 49C. As can be seen in Fig. 3, the total extraction rates of carvone and

limonene were higher at 39C than at 49C. Due to a higher density of SC-CO2 at the lower temperature, the rapid extraction at a lower temperature

318 KIM AND HONG

FIG. 2 Effect of pressure on the extraction of carvone (CV) and limonene (LM) from spearmint

leaves at 39C. The CO2 flow rates were 196, 195, and 192 mL/min at STP for 1000, 1225, and

1500 psi, respectively.

FIG. 3 Effect of temperature on the extraction of carvone (CV) and limonene (LM) from

spearmint leaves at 1500 psi. The CO2 flow rates were 192 and 198 mL/min at STP for 39 and

49C, respectively.

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could be caused by a higher solubility of the oil components in SC-CO2 at a

lower temperature.

Effect of CO2 Flow Rate

Figure 4 shows the effect of performing extraction at different flow rates.

The cumulative masses of carvone and limonene were highly dependent onthe CO2 flow rate. The initial extraction rates were higher with faster exiting

CO2 flow rates. It appears that in the initial period the solvent efficiency (de-

fined by the ratio of cumulative mass of extracted oil to cumulative mass of

passed CO2) decreased with an increase in CO2 flow rate. That is to say, the

extended contact time of CO2 with spearmint leaf particles for the lower flow

rate resulted in higher solvent efficiency. This fact indicates that the internal

mass transfer resistance was more significant than the external mass transfer

resistance in essential oil extraction from spearmint leaves.

Effect of Leaf-Particle Size

As shown in Fig. 5, there exists a significant difference between the initial

extraction rates of two different particle-size leaf samples. The initial extrac-

tion rate of carvone was higher with ground leaves having an average radiusof 0.03 mm than with cut leaves having an average radius of 0.15 mm. It is

likely that a shorter traveling path for the smaller particle size caused faster ex-

traction of carvone in the initial period. This explanation supports the results

from the effect of the CO2 flow rate which showed that internal mass transfer

is more important than external mass transfer.


FIG. 4 Effect of CO2 flow rate on the extraction of carvone (CV) and limonene (LM) from

spearmint leaves at 39C and 1225 psi. The volumetric CO2 flow rates were measured at STP.

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Interestingly, the final yield of limonene from the ground leaves was lower

than what was achieved from cut leaves. This may have been caused by the

loss of limonene during the grinding process as discussed by Goto et al. (9)

concerning the grinding of peppermint leaves. The final yield of carvone from

320 KIM AND HONG

FIG. 5 Effect of leaf-particle size on the extraction of carvone (CV) and limonene (LM) from

spearmint leaves at 39C and 1225 psi. The CO2 flow rates were 96 and 94 mL/min at STP for

the cut and dried leaves, respectively.

FIG. 6 Effect of ethanol modifier on the extraction of carvone (CV) and limonene (LM) from

spearmint leaves at 39C and 1225 psi. The CO2 flow rates were 96, 97, 96, and 96 mL/min at

STP for 0.0, 0.2, 0.5, and 3.0 g ethanol modifier, respectively.

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the ground samples was slightly lower than that from the cut ones, and this

may also be due to loss during the grinding process.

Effect of Ethanol Modifier

Since carvone contains a carbonyl group and limonene does not, ethanol

was used as a modifier to enhance the extraction of carvone rather than

limonene based on polarity. Figure 6 displays the effect of ethanol modifier

added to the spearmint leaves prior to extraction. The ethanol modifier did not

significantly affect the extraction rate of carvone. Instead, large amounts of

undesirable green pigments and waxy materials were extracted when either

0.5 or 3.0 g of ethanol were added. A similar disadvantage from using a mod-

ifier has been reported by Cocero and Calvo (11).

CONCLUSIONS

The essential oil components carvone and limonene were extracted from

spearmint leaves by using SC-CO2. Higher extraction rates were observed at

higher pressures and lower temperatures. Higher extraction rates were ob-

tained at higher CO2 flow rates, but the SC-CO2 solvent efficiency was lower

in the initial period. The initial extraction rate from small particle-size leaves

was higher than that from large particle-size leaves. The addition of ethanol

did not result in any preferential extraction of carvone to limonene.

ACKNOWLEDGMENTS

The authors acknowledge the grant support of the University of California

Energy Institute and the Monsanto Co. K.H.K. acknowledges the scholarship

support of the Ministry of Education, South Korea.

REFERENCES

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2. K. Bauer, D. Garbe, and H. Surburg, Common Fragrance and Flavor Materials, 2nd ed.,

VCH, Weinheim, 1990.3. R. Croteau, F. Karp, K. C. Wagschal, D. M. Satterwhite, D. C. Hyatt, and C. B. Skotland,

Biochemical Characterization of a Spearmint Mutant That Resembles Peppermint in

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of Peppermint and Spearmint,J. Supercrit. Fluids, 5, 157162 (1992).


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7. E. O. Ozer, S. Platin, U. Akman, and O. Hortacsu, Supercritical Carbon Dioxide Extrac-

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Transfer and Optimization,J. Supercrit. Fluids, 5, 256261 (1992).9. M. Goto, M. Sato, and T. Hirose, Extraction of Peppermint Oil by Supercritical Carbon

Dioxide,J. Chem.Eng.Jpn., 26, 401407 (1993).

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Received by editor October 23, 1998

Revision received May 1999

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